A $760 million settlement between the National Football League and thousands of former players, who contend the league downplayed the risk of concussions, was rejected on Tuesday by a U.S. judge who said it might not be enough to pay all of the affected players.

The proposed deal, reached in August, had set aside up to $5 million for each former player diagnosed with a certain brain condition as a result of their years on the playing field.

More than 4,500 former players were named plaintiffs in the lawsuit and up to 20,000 could ultimately be eligible for payment.

"I am primarily concerned that not all retired NFL football players who ultimately receive a qualifying diagnosis, or their related claimants, will be paid," U.S. District Judge Anita Brody wrote in papers filed in federal court in Philadelphia.

"Even if only 10 percent of retired NFL football players eventually receive a qualifying diagnosis, it is difficult to see how the monetary award fund would have the funds available over its lifespan to pay all claimants at these significant award levels," the decision said.

Brody called on the NFL and plaintiffs to submit documentation that they believed showed the money set aside was adequate to meet the potential need.

"We will work with the plaintiffs' attorneys to supply that information promptly to the court and special master," said NFL spokesman Brian McCarthy. "We are confident that the settlement is fair and adequate, and look forward to demonstrating that to the court."

But outside legal observers said the league may need to raise its settlement offer to win the judge's approval.

"They are going to have to come back with a different settlement," said Joseph Farelli, a partner in the New York law firm Pitta & Giblin, which specializes in labor law. "She is saying the amount is not going to cover the people they say are going to be covered by the settlement."

The lawsuit, filed in 2012, contended that the league hid the dangers of brain injury among players while profiting from the sport's sometimes violent physical contact. When the settlement was first disclosed in August, sports business experts described it as a modest amount of money for the NFL, which is believed to generate total annual revenue of $9 billion or $10 billion.

LARGE CLASS

Attorneys for the former players who brought the lawsuit said they believed Brody could yet be convinced that the deal is a good one.

"We are confident that the settlement will be approved after the Court conducts its due diligence on the fairness and adequacy of the proposed agreement," lead attorneys Christopher Seeger of Seeger Weiss LLP and Sol Weiss of Anapol Schwartz said in a statement.

"Analysis from economists, actuaries and medical experts will confirm that the programs established by the settlement will be sufficiently funded to meet their obligations for all eligible retired players."

The judge's move to block the deal reflects her responsibility to ensure that it is fair both to the named plaintiffs and others covered in the class.

"What these 4,500 people are doing is cutting deals for everyone else and it's a one-size fits all deal," said Wayne Dennison, an attorney with Brown Rudnick who specializes in commercial litigation. "The judge has to be sure it's a fair deal not just because the 4,500 like it but because it's a good deal for the full 20,000."

There have been suicides in recent years by current and former NFL players, including Jovan Belcher, Junior Seau, Ray Easterling and Dave Duerson. While their deaths could not be directly connected to the sport, violent or erratic behavior is consistent with symptoms of a condition tied to repeated hits to the head.

A growing body of academic research shows those hits can lead to a condition known as chronic traumatic encephalopathy, which can lead to aggression and dementia.

The research has already prompted the NFL to make changes on the field, including banning the most dangerous helmet-to-helmet hits and requiring teams to keep players who have taken hits to the head off the field if they show certain symptoms including dizziness and memory gaps.